1. Field of the Invention
The present invention relates to acoustic resonators, and more specifically, to film bulk acoustic resonators used in high frequency bands, as well as a manufacturing method for resonators.
2. Description of the Related Art
Recently, wireless communication systems such as mobile telecommunication devices, and high-speed data transfer wireless local area networks (LAN) use high frequency bands which exceed the GHz range. A film bulk acoustic resonator (FBAR) is used as a high frequency element in the high frequency electronic equipment of these types of wireless communication systems. In the past, bulk (ceramic) dielectric resonators, surface acoustic wave elements (SAW) have been used as resonators for high frequency bands. Compared to these resonators, the FBAR is better suited for miniaturization, and has attributes allowing the FBAR to better respond to even higher frequencies. Thus, development is advancing in high frequency filters and resonance circuits using the FBAR.
For the basic structure of the FBAR, a film of piezoelectric material such as aluminum nitride (AlN) or zinc oxide (ZnO) is sandwiched between two electrodes. To attain high performance, a resonator of the FBAR is positioned so as to be suspended over a cavity. For instance, in a stacked cavity FBAR, a sacrificial layer, which serves to build up a structure to be removed in the very last step, is deposited on a support substrate. After processing the sacrificial layer, a bottom electrode, a piezoelectric film, and a top electrode are formed in sequence so as to cover the sacrificial layer. The cavity is then formed in the bottom region of the resonator of the FBAR by removing the sacrificial layer.
AlN film, which is widely used as a piezoelectric film, easily accumulates a high film stress of several hundred MPa to GPa. When stress accumulates on a step region of the stacked cavity FBAR in particular, cracks occur easily. In order to attain desirable piezoelectric properties, the c axis of the hexagonal AlN film is formed so as to be oriented along the direction in which the top and bottom electrodes oppose one another. The orientation of the AlN film changes at the step region of the stacked cavity FBAR. The change in orientation is why there is a problem with the deterioration of piezoelectric properties.
In the process of treating stacked cavity, an edge of the sacrificial layer is given a slant of 20 degrees or less, in order to mitigate the effects of the step region. By doing this, the accumulation of stress on the AlN film deposited on the step region of the slant treated sacrificial layer is mitigated, suppressing the occurrence of cracks. However, providing the slant treatment is very difficult. Disruption of the orientation of the AlN film deposited on the step region of the slant treated sacrificial layer also occurs, leading to a deterioration of piezoelectric properties.
Regarding the above problem, the following method of fabricating an FBAR on a flat substrate surface has been proposed. For instance, after oxidizing a surface of a recess formed on a silicon (Si) substrate, a sacrificial layer is buried into the recess for planarization. Subsequently, a bottom electrode, an AlN film, and a top electrode are formed so as to cover the sacrificial layer, which planarizes the recess. After that, the FBAR is fabricated on a cavity formed by removing the buried sacrificial layer (refer to U.S. Pat. No. 6,060,818).
In another method of fabricating an FBAR (refer to U.S. Pat. No. 6,355,498), a bottom electrode, an AlN film, and a top electrode are formed on an insulating film deposited on a surface of an Si substrate, then a cavity is formed in the underside of the bottom electrode through a via that runs through the AlN layer.
In the FBAR proposed in U.S. Pat. No. 6,060,818, and U.S. Pat. No. 6,355,498, the bottom electrode, the AlN film, and the top electrode are formed over the surface of the flat substrate. Therefore, integrity problems due to stress caused cracks in the AlN film, and problems with deterioration of the piezoelectric properties due to disruption in the orientation of the AlN film are suppressed.
On the other hand, wiring that is connected to the bottom electrode is provided on the insulating layer such as silicon oxide (SiO2), which is provided on the substrate to support the FBAR. Also, the wiring spreading from the top electrode, and the bonding pad, etc. is also provided on the insulating layer on the substrate surface. However, when using the FBAR merged with a semiconductor device such as a complementary metal-oxide-semiconductor (CMOS) circuit on a low resistivity semiconductor substrate, in high frequency band applications in the GHz range, it becomes impossible to overlook the parasitic capacitance between the bonding pad and the wiring on the insulating layer and the low resistivity semiconductor substrate. As a result, the high frequency properties of the FBAR deteriorate.